Blender normals are grey
If you are using Blender Render and a texture with Normal Coordinates (as opposed to the viewport MatCap), the grey pixels apparently come from the image sampling.

In the Texture properties, try setting Extension to 'Clip'.

Or use the OpenGL render with the Normal matcap, which works well.

This happens because when your camera view vector is normal to the faces' normal vectors (in other words, when the face normals are at 90°), the texture is sampled at its border. If the surface is vertical or horizontal (eg. walls and floors) AND the texture repeats, it is sampled at both the left and right borders (respectively top and bottom for floors and ceilings), resulting in an averaged grey.